Convert from UTC date string to Unix timestamp and back

Question

How to convert from string like “2021-11-15 12:10" in UTC to Unix timestamp?

Then add two hours to timestamp (like timestamp += 60*60*2).

Then convert resulting timestamp to string in the same format in UTC?

Using <chrono>, <ctime>, or library, doesn’t really matter.

Answer 1

You could use the I/O manipulators std::get_time and std::put_time.

Example:

#include <ctime>
#include <iomanip>
#include <iostream>
#include <sstream>

int main() {
    std::istringstream in("2021-11-15 12:10"); // put the date in an istringstream
    
    std::tm t{};
    t.tm_isdst = -1; // let std::mktime try to figure out if DST is in effect

    in >> std::get_time(&t, "%Y-%m-%d %H:%M"); // extract it into a std::tm
    
    std::time_t timestamp = std::mktime(&t);   // get epoch

    timestamp += 2 * 60 *60;                   // add 2 hours

    std::tm utc = *std::gmtime(&timestamp);    // make a UTC std::tm
    std::tm lt = *std::localtime(&timestamp);  // make a localtime std::tm
    
    // print the results:
    std::cout << std::put_time(&utc, "%Y-%m-%d %H:%M") << '\n'
              << std::put_time(&lt, "%Y-%m-%d %H:%M") << '\n';
}